Ok. With cbmm the servers check people with the best connection and put them in your lobby. So if im playing say Houston, Tx the game will most likely put me in matches with people who are closer to me like say other Houstonians or people from San Antonio or Dallas. We are close thus make our connection better creating a more stable match. With sbmm, the game picks people based primarily on kd. The majority of players are going to be average as is with every pvp game. So if people with 1.0 kd(average) i.e. you and I jump into a lobby the overwhelming majority of people being average will create a bigger pool of people for us to choose from. Thus having the chance to get people who are closer to us geographically speaking thus creating a better connection. Once you get past 1.5, 2.0, 2.5 kd etc. You get thrown into different pools. The further above average you go the smaller the pool gets. And when the pool gets smaller the game has to reach further to find a player. So if im playing in Houston and have a 2.5kd, my game might have to pull people from California and london depending on who is on at that time and has a general kd as mine. The futher the game has to reach the worse the connection will be. For clarification im not sure what the start and cut off points are for the kd pools, just stating those for example purposes. It could be 1.5 to 2.5 share a pool. Idk bungie hasnt said.